German Patent No. 195 34 445 describes a fuel injector. The fuel injector described in this document has a valve body in which a valve needle is coaxially guided. The valve body has a connection piece through which fuel is supplied to the fuel injector. The valve needle is provided with a central borehole. On the injection side the valve needle forms a seal seat with the valve body. The fuel is supplied to the seal seat via the central borehole of the valve needle. On the outside the valve needle is sealed with respect to the surrounding valve body. A piezoelectric actuator acts upon the valve needle via a pressure shoulder. The pressure shoulder is permanently connected to the valve needle and is tightly guided on the valve body on the inlet side. This protects the actuator against the effect of the fuel pressure. The conventional fuel injector has the following disadvantages:
Because the valve needle is permanently connected to the pressure shoulder, the valve needle on the injection side and the pressure shoulder on the inlet side are sealingly and movably guided in the valve body therefore, manufacturing is relatively complicated and the valve needle of the fuel injector is subject to bending and stresses and the relative positions of the two sliding surfaces are subject to modification.
Because the pressure shoulder, i.e., the valve needle is movably guided with respect to the valve body, the sealing surface is wetted with fuel and, due to the high fuel pressure, the fuel may flow toward the actuator. Thus the actuator is only protected against the effect of the fuel pressure but not against the effect of the fuel. Due to the seal between the pressure shoulder, i.e., the valve needle and valve body, friction losses occur when the fuel injector is actuated. This negatively affects the shapability of the fuel jet. Further, the switching times of the injector are increased, the actuator power is less efficiently utilized, and fuel injector wear is increased. In particular, the seal at the sealing surfaces between the pressure shoulder, i.e., the valve needle and the valve body deteriorates during operation.
Since the central borehole in the valve needle is a part of a fuel line extending from the fuel inlet connection piece to the seal seat, the manufacture of the valve needle is complicated and the fuel injector is subject to dirt deposits, in particular on its seal seat-side end.